In many applications, e.g. of measuring systems with sensors in process measurements technology, especially for determining a measured variable of a process medium, it is desirable to know a failure rate of the measuring systems and to take such into consideration for safety purposes. International standard IEC 61508 gives a method, according to which a failure rate of the total measuring system can be ascertained on the basis of failure rates of individual portions of a measuring system. Among other things, from the ascertained failure rate, there results according to this standard a classification of the measuring system in terms of a Safety Integrity Level, or SIL.
Measuring systems in process measurements technology, especially in the wastewater, drinking water, pharmaceuticals, chemistry and food technology fields, comprise, besides electronic and mechanical components, frequently also electrochemical sensor elements, which often work according to a potentiometric or amperometric measuring principle. Examples of potentiometric sensor types are ion-selective electrodes, of which the most known representative is the glass electrode for pH-value measurement. These sensor types have a measuring half cell and a reference half-cell. The measuring half cell includes an ion-selective membrane, in the case of the pH-glass electrode a glass membrane, which is in contact with the process medium and at which an electrochemical potential forms. The electrochemical potential on the membrane is correlated with the concentration of the ions to be detected, for example, in the case of a pH-glass electrode, H3O+-ions. The potential formed on the membrane is measured against a reference electrode providing a stable reference potential, for example, an Ag/AgCl-electrode, which is in contact with the process medium via a diaphragm.
An example of an amperometric sensor type is a dissolved oxygen sensor according to the principle of the Clark-electrode. This sensor type includes an electrolyte filled measuring chamber, which contains a working- and at least one counterelectrode, and which is isolated from the process medium by means of a membrane permeable for the analyte, e.g. oxygen dissolved in the medium. Analyte diffusing through the membrane is oxidized or reduced on the working electrode, depending on whether the working electrode is connected as anode or cathode. The chemical conversion of the analyte on the working electrode effects an electrical current flow, which is correlated with the analyte concentration in the process medium.
Failure rate is defined as probability of failure referenced to a certain time interval, e.g. probability of failure per hour, or PFH, for short. Failure rates of electronic components are calculatable, or at least estimatable, with the assistance of known tables for most components of a measuring system. Likewise there are, for mechanical components, methods and manners of proceeding, which make such failure rates accessible. In the case of electrical and mechanical components, it is most often known, how the failure rate changes in the case of varying environmental conditions. For example, the change of failure rates of such components with temperature changes can be described on the basis of the Arrhenius equation. Since electronic and mechanical components also, most often, are not directly in contact with the medium, as a rule, only the environmental conditions described by temperature, pressure, humidity and electromagnetic disturbances and oscillations/vibrations are to be noted as influencing variables for the failure rate.
Very much more difficult, however, is the situation in the case of appraisal of electrochemical sensor types, especially in the case of amperometric or potentiometric, sensor types. The process parameters of the process, in which sensor elements of these sensor types are applied, have a strong influence on the life of the sensor elements and therewith also on the probability of failure, or the failure rate, of the corresponding sensor type.
Since the process parameters of the process significantly influence the probability of failure of a sensor element of a determined electrochemical sensor type, a uniform failure rate, i.e. a failure rate unified for all possible processes, such as is given in the case of electronic or mechanical system components, would be of only smaller meaning for an electrochemical sensor element of the specified sensor type. Because of the multitude of conceivable processes, in which sensor elements of one and the same sensor type can be applied, it is, however, also impractical, to determine for each of these processes its own failure rate on the basis of experiments.